Method of manufacturing SOI substrate

ABSTRACT

To easily and accurately flush a substrate surface serving an SOI area with a substrate surface serving as a bulk area, make a buried oxide film, and prevent an oxide film from being exposed on substrate surface. After partially forming a mask oxide film  23  on the surface of a substrate  12  constituted of single crystal silicon, oxygen ions  16  are implanted into the surface of the substrate through the mask oxide film, and the substrate is annealed to form an buried oxide film  13  inside the substrate. Further included is a step of forming a predetermined-depth concave portion  12   c  deeper than substrate surface  12   b  serving as a bulk area on which the mask oxide film is formed on the substrate surface  12   a  serving as an SOI area by forming a thermally grown oxide film  21  on the substrate surface  12   a  serving as an SOI area on which the mask oxide film is not formed between the step of forming the mask oxide film and the step of implanting oxygen ions.

CROSS-REFERENCE TO THE RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/559,347,filed on Nov. 13, 2006, the entire contents of which are incorporatedherein by reference. Also, this application claims the benefit ofpriority under 35 USC 119 to Japanese Patent Application No.2005-333619, filed on Nov. 18, 2005, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing aSilicon-On-Insulator (SOI) substrate partially having a buried oxidefilm in a silicon substrate by a Separation by IMplanted Oxygen (SIMOX)method.

2. Description of the Related Art

It is conventionally expected to use an SOI substrate having a buriedoxide film in a silicon substrate for high-speed low-power-consumptiondevice substrates. Among these device substrates, an SOI substratehaving a buried oxide film in a silicon substrate not entirely butpartially (hereafter referred to as “partial SOI substrate”) is regardedto be important because it is possible to form only a logic portion inan SOI area of a buried oxide film in a system LSI including analoglogic memories and to manufacture a memory portion at a bulk Si portionwithout the buried oxide film.

As this type of method for manufacturing the partial SOI substrate(method for manufacturing SIMOX substrate), the following method (forexample, refer to Patent Document 1) is proposed. More particularly, asshown in FIG. 10, a surface oxide film 4 is first formed on the surfaceof a substrate 2 (substrate 2 is cut in a plane perpendicular to theaxis of a single-crystal silicon rod)(FIG. 10 (a))) to form a patternedresist layer 6 on the surface of the surface oxide film 4 throughphotolithography (FIGS. 10( b) and 10(c)). Then, the surface oxide film4 is patterned through anisotropic etching (FIGS. 10( d) and 10(e)) toremove the resist layer 6 (FIG. 10F) and then, the substrate 2 iscleaned. Then, oxygen ions 7 are implanted into the surface of thesubstrate 2 (FIG. 10( g)) and the substrate 2 is immersed in a mixedsolution (etching solution) of hydrofluoric-acid ammonium aqueoussolution and hydrofluoric acid to remove the surface oxide film 4 (FIG.10( h)). Moreover, the substrate 2 is held at 1,300° C. or higher forpredetermined time in the atmosphere of mixed gas of argon and oxygen ornitrogen and oxygen and is annealed to form a buried oxide film 3 (FIG.10( i)), then the substrate 2 is immersed in a mixed solution (etchingsolution) of hydrofluoric-acid ammonium aqueous solution andhydrofluoric acid to remove the surface oxide film 4 (FIG. 10( j)).

Moreover, as shown in FIG. 11, the surface oxide film 4 is formed on thesurface of the substrate 2 (substrate 2 is cut in a plane perpendicularto the axis of single-crystal silicon rod)(FIG. 11( a)) and a patternedresist layer 6 is formed on the surface of the surface oxide film 4through photolithography (FIGS. 11B(b) and 11C(c)). Then, the surfaceoxide film 4 is patterned through anisotropic etching (FIGS. 11( d) and11(e)) to remove the resist layer 6 (FIG. 11( f)) and then, thesubstrate 2 is cleaned. Then, oxygen ions 7 are implanted into thesurface of the substrate 2 (FIG. 11( g)), the substrate 2 is held forpredetermined time at 1300° C. or higher in the atmosphere of mixed gasof argon and oxygen or mixed gas of nitrogen and oxygen and is annealedto form the buried oxide film 3 (FIG. 11( h)). Furthermore, thesubstrate 2 is immersed in mixed solution (etching solution) ofhydrofluoric-acid ammonium aqueous solution and hydrofluoric acid toremove the surface oxide film 4 (FIG. 11( i)).

However, as shown in FIG. 11( g), when implanting oxygen ions 7 into thesubstrate 2, the upside of the surface oxide film 4 left on the surfaceof the substrate 2 as a mask is expanded and deformed due toimplantation of oxygen ions 7 and thus the buried depth at the end ofthe buried oxide film 3 is decreased. Then, an edge area is exposed onthe surface of the substrate 2 because the end of the buried oxide film3 becomes thicker at the time of subsequent annealing. When removing thesurface oxide film 4, the edge area is etched, which occasionally causesa concave groove 8 to be formed on the surface of the substrate 2 alongthe edge area or cavity to be formed on the buried oxide film 3. Todissolve this point, it is proposed to vertically apply anisotropicetching to the surface of the substrate by using the surface oxide filmas a mask and vertically implant oxygen ions into the surface of thesubstrate (for example, refer to Patent Document 2). In this method formanufacturing An SOI substrate, since oxygen ions are implanted in thedirection vertical to the surface of a substrate, the edge area of aburied oxide film is not exposed on surface and even if immersing thesubstrate in an etching solution in order to remove a surface oxidefilm, the edge area of the buried oxide film is not removed. Therefore,it is possible to prevent an unnecessary concave groove from beingformed on the surface of the substrate or unnecessary cavity from beingformed on the buried oxide film.

[Patent Document 1] Japanese Patent Laid-Open No. H5-82525 (claim 2)

[Patent Document 2] Japanese Patent Laid-Open No. 2001-308025 (claim 1)

SUMMARY OF THE INVENTION

However, in the above conventional method for manufacturingSIMOX-substrate described in Patent Document 1, as shown in FIGS. 10( i)and 10(j), an oxygen ion area 9 serving as the buried oxide film 3 isexpanded in volume at the time of annealing after implanting oxygen ions7. This expansion further causes a substrate surface 2 a serving as anSOI area to be expanded in more volume than a substrate surface 2 bserving as a bulk area. As a result, a trouble that a step is formed onthe surface of the substrate 2 after removing a surface oxide layer 8occurs.

Moreover, in the conventional method described in Patent Document 1, asshown in FIG. 10( g), a recess portion 2 c is partially formed on thesubstrate surface 2 a serving as an SOI area due to sputtering of oxygenions 7 when implanting oxygen ions 7. This presents fears that thethickness of the oxide film 3 after annealed is partially changed or theburied oxide film 3 is occasionally exposed on the surface of thesubstrate 2 after annealed as shown in FIG. 10( j).

Furthermore, in the above conventional method described in PatentDocument 1, the periphery of the upper margin of a surface oxide filmserving as a mask when implanting oxygen ions is occasionally deformedso as to be expanded. This deformation presents a fear that when a partof the surface oxide film is protruded to a portion serving as an SOIarea not covered with the surface oxide film, a fear is presented thatthe implantation depth of oxygen ions to be implanted may be changed.

Furthermore, in the above method described in Patent Document 1, thereis a problem that the peripheral portion of an oxygen ion area servingas a buried oxide film is more oxidized than the central portion of theoxygen ion area at the time of annealing after implanting oxygen ionsand expanded in volume. That is, though the peripheral portion of oxygenions implanted into the surface of the substrate is not exposed on thesurface of the substrate after the ions are implanted, oxygen issupplied to the peripheral portion of an oxygen ion area from thesurface as well as from the periphery at the time of subsequentannealing periphery. Therefore, more quantity of oxygen is supplied tothe peripheral portion of the oxygen ion area as compared that to thecentral portion to which oxygen is supplied from only the surfacedirection and oxidation at the peripheral portion is more advanced ascompared at the central portion at the time of annealing and thus theperipheral portion of the buried oxide film after annealed is increasedin thickness as compared to the central portion. When increase in thethickness advances, there is a problem that the peripheral portion isexposed on the surface of the substrate.

Meanwhile, in the method for manufacturing an SOI substrate described inPatent Document 2, even if vertically implanting oxygen ions into thesurface of the substrate, a problem to be solved is still left thatoxidation at the peripheral portion of a buried oxide film is moreadvanced than at other portion. That is, peripheryoxygen ions implantedvertically into the surface of the substrate is not exposed on thesurface of the substrate after the ions are implanted. However, oxygenis supplied to the periphery of the buried oxide film from the surfaceas well as from the periphery of the film at the annealing. Therefore,more quantity of oxygen is supplied as that compared to the centralportion to which oxygen is supplied only from the surface direction sothat oxidation at the peripheral portion is further advanced than at thecentral portion at the time of annealing, and thus the periphery of theburied oxide film after annealed is increased in thickness as comparedat the central portion. When increase in the thickness advances, thereis a trouble that the periphery of the buried oxide film is exposed onthe substrate surface.

Moreover, in the conventional method described in Patent Document 2, itis known that the periphery of the upper margin of a surface oxide filmserving as a mask when oxygen ions are implanted is deformed so as to beexpanded. When a part of a surface oxide film is protruded to a portionnot covered with the surface oxide film, a fear is presented that theimplantation depth of oxygen ions to be implanted may be changed.Therefore, it is also necessary to effectively prevent the edge area ofthe obtained buried oxide film from being exposed on the surface of asubstrate by inhibiting deformation in which a part of the surface oxidefilm is protruded to a portion not covered with the surface oxide filmand uniforming the depth of the buried oxide film when oxygen ions areimplanted.

It is the first object of the present invention to provide a method formanufacturing an SOI substrate capable of easily and accurately making asubstrate surface serving as an SOI area flush with a substrate surfaceserving as a bulk area.

It is the second object of the present invention to provide a method formanufacturing an SOI substrate capable of uniforming the thickness of aburied oxide film and effectively preventing the buried oxide film frombeing exposed on substrate surface.

It is the third object of the present invention to provide a method formanufacturing an SOI substrate capable of uniforming the implantationdepth of oxygen ions by preventing expansion and deformation of theperiphery of the upper margin of a mask oxide film when oxygen ions areimplanted.

It is the fourth object of the present invention to provide a method formanufacturing an SOI substrate capable of securely preventing theperiphery of a buried oxide film from being exposed on substrate surfaceby preventing oxygen from a boundary area contacting with the side of amask oxide film on the substrate surface serving as an SOI area.

As shown in FIG. 1, the invention of claim 1 is an improved method formanufacturing an SOI substrate including a step of partially forming amask oxide film 23 on the surface of a substrate 12 made of singlecrystal silicon, a step of implanting oxygen ions 16 into the surface ofthe substrate 12 through the mask oxide film 23, and a step of annealingthe substrate 12 to form a buried oxide film 13 inside the substrate 12.

The invention of claim 1 is characterized in further including a step offorming recess portion 12 c with a predetermined-depth deeper than thesurface of a substrate 12 b serving as a bulk area on which the maskoxide film 23 is formed on the surface of a substrate 12 a serving as anSOI area on which a mask oxide film 23 is not formed by forming athermally grown oxide film 21 between the step of forming the mask oxidefilm 23 and the step of implanting oxygen ions 16.

In the method described in claim 1, it is possible to form recessportion 12 c with a predetermined-depth deeper than the substratesurface 12 b serving as a bulk area on the substrate surface 12 aserving as an SOI area by forming a thermally-oxidized film 21 on thesubstrate surface 12 a between a step of forming a mask oxide film 23and a step of implanting oxygen ions 16. As a result only the substratesurface 12 a serving as an SOI area is lifted due to volume expansion ofan area 20 of oxygen ions implanted into the substrate 12 and thus thesubstrate surface 12 a becomes flush with the substrate surface 12 b.Moreover, because the substrate surface 12 a serving as an SOI area iscovered with the above thermally grown oxide film 21, it is possible toprevent the substrate surface 12 a serving as an SOI area due tosputtering generated along with oxygen ion implantation from beingpartially etched.

As shown in FIG. 2, the invention of FIG. 2 is characterized in furtherincluding between a step of forming the mask oxide film 23 and a step ofimplanting oxygen ions 16, a step of forming a buffer film 32 on asubstrate surface 12 a serving as an SOI area on which a mask oxide film23 is not formed, and on the upside and the side of the mask oxide film.

In the method described in FIG. 2, included is a step that the bufferfilm 32 is formed on the substrate surface 12 a on which the mask oxidefilm 23 is not formed, and on the upside and the side of the mask oxidefilm 23 between the step of forming the mask oxide film 23 and the stepof implanting oxygen ions 16. As a result, though sputtering occursalong with implantation of oxygen ions 16, because the substrate surface12 a serving as an SOI area is covered with the buffer film 32, it ispossible to prevent the substrate surface 12 a serving as an SOI areafrom being partialpartially etched through sputtering. Moreover, becausethe buffer film 32 formed on the side of the mask oxide film 23 preventsthe upper margin of the mask oxide film 23 from being deformed so as toexpand when implanting oxygen ions 16, it is possible to uniform theimplantation depth of oxygen ions 16.

As shown in FIG. 3, the invention of FIG. 3 is characterized in furtherincluding between a step of implanting oxygen ions 16 and a step ofperforming annealing the substrate 12, a step of etching a mask oxidefilm 23 so as to become a predetermined thickness and a step of forminga buffer film 42 on a predetermined-width boundary area contacting withthe side of the mask oxide film 23 on a silicon substrate surface 12 aserving as an SOI area and on the side of the mask oxide film 23.

The method described in FIG. 3 further includes a step of etching themask oxide film 23 so as to become a predetermined thickness and formingthe buffer film 42 on a predetermined-width boundary area contactingwith the side of the mask oxide film 23 of a silicon substrate surface12 a serving as an SOI area and on the side of the mask oxide film 23.In a case of annealing the boundary area contacting with the side of themask oxide film 23 decreased in thickness on the substrate surface 12 aserving as an SOI area and the side of the mask oxide film 23 decreasedin thickness while they are exposed without being covered with thebuffer film 42, oxidation is further advanced than at the centralportion at the time of annealing, and thus the peripheral portion of aburied oxide film 13 after annealed is increased in thickness and thereis a fear that the peripheral portion may be exposed on substratesurface 12 a. However, in the invention of claim 3, the boundary area inthe substrate surface 12 a serving as an SOI area, contacting with theside of the mask oxide film 23 decreased in thickness and the side ofthe mask oxide film 23 decreased in thickness are covered with a bufferfilm 42, so that it is possible to prevent oxygen from entering from theboundary area. As a result, it is possible to securely prevent theperiphery of the buried oxide film 13 from being exposed on the surfaceof the substrate 12.

As shown in FIG. 4, the invention of FIG. 4 is characterized in furtherincluding between a step of implanting oxygen ions 16 and a step ofperforming annealing the substrate 12, a step of forming a buffer film52 on a predetermined-width boundary area contacting with the side of amask oxide film 23 on a silicon substrate surface 12 a serving as an SOIarea and on the side of the mask oxide film 23 and a step of etching themask oxide film 23 so as to become a predetermined thickness.

The method described in FIG. 4 further includes a step of forming thebuffer film 52 on the predetermined-width boundary area contacting withthe side of the mask oxide film 23 on the silicon substrate surface 12 aserving as an SOI area and on the side of the mask oxide film 23 and astep of etching the mask oxide film 23 so as to become a predeterminedthickness between the step of implanting oxygen ions 16 and the step ofannealing the substrate 12. In a case of annealing the boundary areacontacting with the side of the mask oxide film 23 decreased inthickness of the substrate surface 12 a serving as an SOI area and theside of the mask oxide film 23 while they are exposed without beingcovered with a buffer film 42, oxidation at the peripheral portion isfurther advanced than at the central portion and thus the peripheralportion of a buried oxide film 13 is increased in thickness and there isa fear the peripheral portion may be exposed on the substrate surface 12a. However, in the invention of claim 4, because the boundary areacontacting with the side of the mask oxide film 23 decreased inthickness of the substrate surface 12 a serving as an SOI area and theside of the mask oxide film 23 decreased in thickness are covered withthe buffer film 52, even if isotropic etching easier than anisotropicetching is used for decrease of the mask oxide film 23 in thicknessinstead of using a resist layer, the side of the mask oxide film 23 isnot etched, so that it is possible to prevent oxygen from entering fromthe boundary area. As a result, it is possible to securely prevent theperiphery of the buried oxide film 13 from being exposed on the surfaceof the substrate 12.

As shown in FIG. 5, the invention of FIG. 5 is an improved method formanufacturing an SOI substrate including a step of partially forming amask oxide film 23 on the surface of a silicon substrate 12, a step ofimplanting oxygen ions 16 into the surface of the substrate 12 throughthe mask oxide film 23, and a step of annealing the substrate 12 to forma buried oxide film 13 inside the substrate 12.

The invention of FIG. 5 is characterized in further including a step ofimplanting silicon ions 62 into the surface of a silicon substrate 12corresponding to the periphery of a buried oxide film 13 to be formedbefore forming the mask oxide film 23 or between a step of forming themask oxide film 23 and a step of implanting oxygen ions 16.

In the method described in FIG. 5, silicon ions 62 are implanted intothe surface of the silicon substrate 12 corresponding to the peripheryof the buried oxide film 13 to be formed before implanting oxygen ions.Therefore, the silicon ions inhibit oxidation around the buried oxidefilm at the time of subsequent annealing so that avoided is a situationin which thickness is further increased as compared to other portion. Asa result, it is prevented that an edge area on the periphery of theburied oxide film due to the expansion is exposed on the surface of thesubstrate.

As shown in FIG. 6, the invention of claim 7 is an improved method formanufacturing an SOI substrate including a step of partially forming amask oxide film 23 on the surface of a silicon substrate 12, a step ofimplanting oxygen ions 16 into the surface of the substrate 12 throughthe mask oxide film 23, and a step of annealing the substrate 12 to forma buried oxide film 13 inside the substrate 12.

The invention of FIG. 6 is characterized in further including a step offorming a concave groove 72 on the surface of a silicon substrate 12corresponding to the periphery of a buried oxide film 13 to be formedbefore forming a mask oxide film 23 or between a step of forming themask oxide film 23 and a step of implanting oxygen ions 16.

In the method of FIG. 6, since a concave groove 72 is formed on thesurface of the silicon substrate 12 corresponding to the periphery ofthe buried oxide film 13 to be formed before implanting oxygen ions, theperiphery of the buried oxide film obtained through implantation ofoxygen ions is kept at a distance from the surface of the substrate 12along the concave groove 72. Therefore, even if the thickness around theburied oxide film is further increased than other portion at the time ofannealing, the edge area on the periphery of the buried oxide film doesnot reach the surface of the substrate nor is exposed on the surface.

As shown in FIG. 7, the invention of FIG. 7 is an improved method formanufacturing an SOI substrate including a step of partially forming amask oxide film 23 on the surface of a silicon substrate 12, a step ofimplanting oxygen ions 16 into the surface of a substrate 12 through themask oxide film 23, and a step of annealing the substrate 12 to form aburied oxide film 13 inside the substrate 12.

The invention of FIG. 7 is characterized in that the implantation ofoxygen ions 16 is performed a plurality of times separately and thepresent invention further includes a step of etching the margin of amask oxide film 23 between the precedent oxygen-ion implanting step andthe subsequent oxygen-ion implanting step.

As shown in FIG. 8, the present invention of FIG. 8 is an improvedmethod for manufacturing an SOI substrate including a step of partiallyforming a mask oxide film 23 on the surface of a silicon substrate 12, astep of implanting oxygen ions 16 into the surface of the substrate 12through the mask oxide film 23, and a step of annealing the substrate 12to form a buried oxide film 13 inside the substrate 12.

The invention of FIG. 8 is characterized in that the implantation ofoxygen ions 16 is performed a plurality of times separately and thepresent invention further including a step of enlarging the margin of amask oxide film 23 between the precedent oxygen-ion implanting step andthe subsequent oxygen-ion implanting step.

In the methods described in FIG. 8, Since between each step ofimplanting oxygen ions 16 a plurality of times separately, the margin ofthe mask oxide film 23 is decreased by etching or enlarged by overlayingto the periphery of the mask oxide film 23 to expand the periphery, theperiphery of a buried oxide film obtained through implantation of oxygenions a plurality of times becomes thinner than other portions.Therefore, even if the thickness at the periphery of the buried oxidefilm is further increased as compared to other portions at the time ofsubsequent annealing, and thus the edge area on the periphery of theburied oxide film does not reach the surface of a substrate nor isexposed on the surface.

As shown in FIG. 9, the invention of FIG. 9 is an improved method formanufacturing an SOI substrate including a step of partially forming amask oxide film 23 on the surface of a silicon substrate 12, a step ofimplanting oxygen ions 16 into the surface of the substrate 12 throughthe mask oxide film 23, and a step of annealing the substrate 12 to forma buried oxide film 13 inside the substrate 12.

The invention of FIG. 9 is characterized in that the step of forming themask oxide film 23 includes a step of forming a surface oxide film 14 onthe surface of the substrate 12, a step of forming a resist layer 17having a predetermined pattern on the surface of the surface oxide film14, a step of applying isotropic etching to the surface oxide film 14 byusing the resist layer 17 as a mask to decrease the thickness of thesurface oxide film 14 not masked with the resist layer 17, a step ofremoving the surface oxide film 14 decreased in thickness because ofvertically applying anisotropic etching to the surface of the substrate12 by using the resist layer 17 as a mask, and a resist removing step ofremoving the resist layer 17 and making the surface oxide film 14partially left on the surface of the substrate 12 as the mask oxide film23 to form a recess portion 23 a on the upper corner of the obtainedmask oxide film 23.

In the method described in FIG. 9, Since the recess portion 23 a isformed on the upper corner of the mask oxide film 73, even if the uppermargin of the mask oxide film 73 is deformed so as to be expanded whenimplanting oxygen ions, the deformed portion is not protruded to aportion not covered with the mask oxide film 73 and thus implantationdepth of implanted oxygen ions is not changed. Therefore, the depth ofthe buried oxide film obtained through oxygen ion implantation becomesuniform and the edge area of the buried oxide film can effectivelyprevent the edge area of the buried oxide film from being exposed on thesurface of the substrate.

As described above, according to the present invention, by forming athermally grown oxide film on the substrate surface serving as an SOIarea on which the mask oxide film is not formed between the step offorming the mask oxide film and the step of implanting oxygen ions, arecess portion with a predetermined-depth deeper than the substratesurface serving as a bulk area on which the mask oxide film is formed isformed on the substrate surface serving as an SOI area. Therefore, onlythe substrate surface serving as an SOI area is lifted due to the volumeexpansion of the area of oxygen ions implanted into the substrate at thetime of annealing, and the substrate surface serving as an SOI areabecomes flush with the substrate surface serving as a bulk area. As aresult, it is possible to make the substrate surface serving as an SOIarea and the substrate surface serving as a bulk area easily andaccurately flush so that it is possible to prevent a focus from beingshifted in a subsequent photolithography step of an SOI substrate.Moreover, because the substrate surface serving as an SOI area iscovered with the above thermally grown oxide film, it is possible toinhibit the substrate surface serving as an SOI area from beingpartially etched due to sputtering along with oxygen ion implantation.As a result, it is possible to uniform the thickness of the buried oxidefilm nor is exposed on the substrate surface. Therefore, because theburied oxide film is not etched when etching an oxide layer on thesurface of the substrate formed after annealing, a hole serving as aparticle occurrence source is not formed on the substrate. Moreover,because it is possible to simultaneously form the recess portion andthermally grown oxide film only by the comparatively simple treatmentreferred to as thermal oxidation, only a small number of man hours forforming the recess portion and thermally grown oxide film is required.

Moreover, by including a step of forming a buffer film on the substratesurface serving as an SOI area on which a mask oxide film is not formedand on the upside and the side of the mask oxide film, the substratesurface serving as an SOI area is covered with the buffer film so thatit is possible to prevent the substrate surface serving as an SOI areafrom being partially etched due to sputtering along with oxygen ionimplantation. Moreover, because the buffer film formed on the side ofthe mask oxide film presents the upper margin of the mask oxide filmfrom being deformed so as to be expanded when implanting oxygen ions, itis possible to uniform the implantation depth of oxygen ions.

Furthermore, by including a step of forming a buffer film-on thepredetermined-width boundary area contacting with the side of a maskoxide film on the silicon substrate surface serving as an SOI area andon the side of the mask oxide film after etching the mask oxide film toform the predetermined thickness between a step of implanting oxygenions and a step of performing annealing, the predetermined-widthboundary area contacting with the side of the mask oxide film on thesubstrate surface serving as an SOI area and the side of the mask oxidefilm decreased in thickness are covered with the buffer film so that itis possible to prevent oxygen from entering from the boundary area. As aresult, it is possible to securely prevent the periphery of the buriedoxide film from being exposed on the surface of the substrate.

Furthermore, between a step of implanting oxygen ions and a step ofperforming annealing, by forming a buffer film on thepredetermined-width boundary area contacting with the side of a maskoxide film on the silicon substrate surface serving as an SOI area andthe side of the mask oxide film and then, by etching the mask oxide filmto form a predetermined thickness the boundary area contacting with theside of the mask oxide film decreased in thickness on the substratesurface serving as an SOI area and the side of the mask oxide film arecovered with the buffer film so that, even if isotropic etching easierthan anisotropic etching for decrease in thickness of the mask oxidefilm without using a resist layer is used, because the side of the maskoxide film is not etched, it is possible to prevent oxygen from enteringfrom a boundary area As a result, it is possible to securely prevent theperiphery of the buried oxide film from being exposed on the surface ofthe substrate.

Silicon ions are implanted into the surface of a silicon substratecorresponding to the periphery of a buried oxide film to be obtained ora concave groove is formed on the surface of the silicon substratebefore forming a mask oxide film or between a step of forming the maskoxide film and a step of implanting oxygen ions. When implanting siliconions, it is possible to avoid the situation that the silicon ionsinhibit oxidation around the buried oxide film at the time of subsequentannealing so that the thickness of the periphery is further increased ascompared to other portion. When forming a concave groove, it is possibleto keep the periphery of a buried oxide film obtained through oxygen ionimplantation at a distance from the surface of the substrate. As aresult, it is possible to prevent the edge area around the buried oxidefilm from reaching the surface of the substrate and being exposed on thesurface of the substrate.

Moreover, when performing oxygen ion implantation a plurality of timesseparately, it is possible to decrease the periphery of a buried oxidefilm in thickness as compared to other portion by etching and decreasingthe margin of a mask oxide film or by weld-overlaying and enlarging theperiphery of the mask oxide film between a precedent oxygen-ionimplanting step and a subsequent oxygen-ion implanting stepperiphery.Thus, by decreasing the periphery of the oxide film obtained throughoxygen ion implantation in thickness as compared to other portion, it ispossible to prevent that the edge area on the periphery of the buriedoxide film is exposed on the surface of the substrate due to thethickness of the periphery of the buried oxide film further increased ascompared to other portion at the time of subsequent annealing.

Furthermore, by forming a recess portion on the upper corner of the maskoxide film, even if the upper margin on the mask oxide film is deformedso as to be extended at the time of oxygen ion implantation, thedeformed portion is not protruded to a portion not covered with the maskoxide film and thus the implantation depth of implanted oxygen ions isnot changed. Therefore, the thickness of the buried oxide film obtainedtrough oxygen ion implantation becomes uniform and it is possible toprevent the edge area of the buried oxide film from being exposed on thesurface of a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is sectional view showing the method for manufacturing an SOIsubstrate of first embodiment of the present invention in order of step;

FIG. 2 is sectional view corresponding to FIG. 1, showing secondembodiment of the present invention;

FIG. 3 is sectional view corresponding to FIG. 1, showing thirdembodiment of the present invention corresponding to FIG. 1;

FIG. 4 is sectional view corresponding to FIG. 1, showing fourthembodiment of the present invention;

FIG. 5 is sectional view corresponding to FIG. 1, showing fifthembodiment of the present invention;

FIG. 6 is sectional view corresponding to FIG. 1, showing sixthembodiment of the present invention;

FIG. 7 is sectional view corresponding to FIG. 1, showing seventhembodiment of the present invention;

FIG. 8 is sectional view corresponding to FIG. 1 showing eighthembodiment of the present invention;

FIG. 9 is sectional view corresponding to FIGS. 1A to 1K, showing ninthembodiment of the present invention;

FIG. 10 is sectional view corresponding to FIG. 1, showing aconventional example; and

FIG. 11 is sectional view corresponding to FIG. 1, showing anotherconventional example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Then, preferred embodiments of the present invention are described belowby referring to the accompanying drawings.

First Embodiment

As shown in FIG. 1( k), an SOI substrate 11 has a silicon substrate 12and a buried oxide film 13 formed inside the substrate 12. The substrate12 is cut in a sliced form along a plane perpendicular to the axis of asingle-crystal silicon rod grown by Czochralski (CZ) method [plane (100)of a crystal structure of a single crystal silicon]. Moreover, a buriedoxide film 13 is formed as described below. It is allowed to cut asubstrate from a single-crystal silicon rod or single-crystal siliconplate grown not by the CZ method but by the floating zone (FZ) method orthe like.

Firstly, a surface oxide film 14 is formed on the surface of thesubstrate 12 (FIG. 1( a)). This surface oxide film 14 is a silicon oxidefilm (SiO₂ film), which is formed by thermally oxidizing the substrate12 or with the CVD method (chemical vapor deposition method). Moreover,the surface oxide film 14 is formed in the thickness range between 200and 1,000 nm, preferably in the thickness range between 500 and 800 nm.This limitation of the thickness of the surface oxide film 14 in therange between 200 and 1,000 nm is because there is a fear that oxygenions 16 to be described later may pass through the surface oxide film 14and be implanted into the substrate 12 and because it is possible tosufficiently cut off the oxygen ions 16 at 1,000 nm or less. Then, aresist layer 17 having a predetermined pattern is formed on the surfaceof the surface oxide film 14 through photolithography (FIGS. 1( b) and1(c)). The predetermined pattern is formed on the resist layer 17 byexposing the resist layer 17 by a photomask 18 (FIG. 1( b)) and goingthrough development and rinsing (FIG. 1( c)).

Anisotropic etching is vertically applied to the surface oxide film 14for the surface of the substrate 12 by using the above resist layer 17as a mask (FIG. 1( d)). The anisotropic etching is reactive ion etchingin this embodiment. In the reactive ion etching, though not illustrated,by mounting a substrate on the lower electrode of two oppositeelectrodes set in a reactive chamber and applying a high-frequencyvoltage to these electrodes to induce plasma, radical-ion nuclearspecies higher in reactivity than etching gas such as CF₄ or SF₆ isformed, the radical ions at several tens to several hundreds of eV enterthe substrate 12, and etching of the surface oxide film 14 is advancedby both effects of sputtering action and chemical reaction due to theradical ions. Therefore, the inner margin of the surface oxide film 14becomes a vertical etching shape free from undercut. Incidentally, It isallowed to use ECR plasma etching as anisotropic etching. After theetching is completed, the resist layer 17 is removed by a sulfuricacid/hydrogen peroxide solution or the like, a mask oxide film 23 formedof the surface oxide film 14 left on the surface of the substratewithout being etched and having a thickness of 200 to 1,000 nm ispartially formed on the surface of the substrate 12 (FIG. 1( e)) andthen cleaned.

Then, a recess portion 12 c with the depth deeper than the substratesurface 12 b serving as a bulk area on which the mask oxide film 23 isformed on the substrate surface 12 a serving as an SOI area on which themask oxide film 23 is not formed. In this embodiment, a step of formingthe predetermined-thickness thermally grown oxide film 21 on thesubstrate surface 12 a serving as an SOI area corresponds to a step offorming the thermally grown oxide film 21 on the substrate surface 12 aserving as an SOI area (FIG. 1( f)). That is, by thermally oxidizing thesubstrate surface 12 a serving as an SOI area and forming the thermallygrown oxide film 21 on the substrate surface 12 a, a recess portion 12 cwith a predetermined-thickness deeper than the substrate surface 12 bserving as a bulk area on which the mask oxide film 23 is formed. Thedepth of the recess portion 12 c is equivalent to the volume expansionof the buried oxide film 13 formed after the annealing to be describedlater and is previously obtained through an experiment. Specifically,the depth of the above 12 c is the difference between the thickness ofthe area 20 of oxygen ions implanted into the substrate 12 and thethickness of the buried oxide film 13 formed when the oxygen ion area 20is expanded in volume through annealing and is 30 to 80% of thethickness of the buried oxide film 13 after annealed, preferably 55%.The buried oxide film 13 is generally formed with a predeterminedthickness in the range between 20 and 200 nm. However, when thethickness of the buried oxide film 13 is 20 nm, the depth of the aboverecess portion 12 c is 6 to 10 nm, preferably 11 nm. When the thicknessof the buried oxide film 13 is 200 nm, the thickness of the above recessportion 12 c is 105 to 115 nm, preferably into 110 nm. In this case, thethickness of the above recess portion 12 c is limited to the range of 30to 80% because it is possible to implant oxygen ions 16 into thesubstrate 12 through the above thermally grown oxide film 21 and inorder to eliminate a step formed between the substrate surface 12 aserving as an SOI area and the substrate surface 12 b serving as a bulkarea after annealed. Though a state completely free from the above stepis the best, a slight step may occur due to an error of a targetthickness of the thermally grown oxide film 21 or to a target thicknessof the buried oxide film 13. When the step is 30 nm or less, it ispossible to prevent a focus from being shifted in a subsequentphotolithography step.

Then, oxygen ions 16 are implanted into the surface of the substrate 12by using the mask oxide film 23 as a mask (FIGS. 1( g) and 1(h)). In theimplanting condition of oxygen ions 16, the implanting quantity rangesbetween 1×10¹⁷/cm² and 2×10¹⁸/cm² ₇ preferably between 2×10¹⁷/cm² and5×10¹⁷/cm² and the implantation energy ranges between 20 keV and 200keV, preferably between 60 keV and 180 keV. After implanting oxygen ions16, the mask oxide film 23 and thermally grown oxide film 21 are removedfrom the surface of the substrate 12 through wet etching (isotropicetching) (FIG. 1( i)). Thereby, the above recess portion 12 c is exposedon the substrate surface 12 a serving as an SOI area. Though sputteringoccurs along with oxygen ion implantation when implanting oxygen ions,the substrate surface 12 a serving as an SOI area is covered with thethermally grown oxide film 21 so that it is possible to prevent thesubstrate surface 12 a serving as an SOI area from being partiallyetched due to sputtering. After implanting oxygen ions, the substrate 12is immersed in a mixed solution of hydrofluoric-acid ammonium aqueoussolution and hydrofluoric acid (etching solution) to remove the maskoxide film 23 and thermally grown oxide film 21 from the surface andmoreover, annealing where the substrate 12 is held in an oxidizingatmosphere at 1300 to 1380° C. for 2 to 20 hours and then slowly colled(FIG. 1( j)). The oxidizing atmosphere is exemplified by a mixed gasatmosphere of argon and oxygen or mixed gas atmosphere of nitrogen andoxygen including a mixed gas atmosphere of inert gas and oxygen. In thiscase, the oxidizing atmosphere contains 100 volume percent of oxygen.Preferable content of oxygen ranges between 0.5 and 90 volume percentand more preferable content ranges between 40 and 70 volume percent.This is because it is impossible to expect oxidation on the surface ofthe substrate 12 at the time of annealing to be described later when theoxygen percentage content is less than 0.5%.

Oxidation of the oxygen ion area 20 of the substrate 12 is acceleratedby the annealing and the buried oxide film 13 is formed inside thesubstrate 12. When forming the buried oxide film 13, an oxygen ion areaserving as the buried oxide film 13 is expanded in volume, only thesubstrate surface 12 serving as an SOI area is expanded and lifted so asto embed the recess portion 12 c, and the substrate surface 12 a servingas an SOI area becomes flush with the substrate surface 12 b serving asa bulk area. At the same time, an oxidized layer 22 due to annealing isformed on the surface of the substrate 12. After forming the buriedoxide film 13 through the above annealing, the substrate 12 is immersedin a mixed solution of hydrofluoric-acid ammonium aqueous solution andhydrofluoric acid (etching solution) to remove the oxidized layer 22(FIG. 1( k)). Then, the surface of the SOI substrate 11 becomes flatwithout step. Thus, it is possible to easily and accurately make thesubstrate surface 12 serving as an SOI area and substrate surface 12 bserving as a bulk area after annealed flush. Thereby, it is possible toprevent the focus of the above SOI substrate 11 from being shifted evenif exposing the SOI substrate 11 in a photolithography step.

As described above, by filling the above recess portion 12 c with thethermally grown oxide film 21 formed through thermal oxidation between astep of forming the recess portion 12 c and a step of implanting oxygenions 16, it is possible to prevent the substrate surface 12 a serving asan SOI area through the sputtering generated along with oxygen ionimplantation from being partially etched so that it is possible touniform the thickness of the buried oxide film 13 and the buried oxidefilm 13 is not exposed on the surface of the substrate 12. As a result,the buried oxide film 13 is not etched when etching the oxidized layer22 on the surface of the substrate 12 formed through annealing and thusa hole serving as a particle generating source is not formed on thesubstrate 12.

Second Embodiment

FIG. 2 shows second embodiment of the present invention. In FIG. 2, anumeral same as that in FIG. 1 denotes the same component.

This embodiment further includes a step of forming a buffer film 32 onthe substrate surface 12 a serving as an SOI area where a mask oxidefilm 23 is not formed and on the upside and the side of a mask oxidefilm 23. Specifically, a buffer film 32 made of silicon nitride isformed on the substrate surface 12 a serving as an SOI area and theupside and the side of the mask oxide film 23 after verticallyanisotropically etching a surface oxide film 14 vertically to thesurface of a substrate 12 by using a resist layer 17 as a mask (FIG. 2(d)) before implanting oxygen ions 16 (FIG. 2( f)). The thickness of thebuffer film 32 ranges between 5 and 500 nm, preferably between 20 and200 nm. This limitation of the thickness of the buffer film 32 in therange of 5 to 500 nm because it is impossible to prevent oxygen fromentering from a boundary area at the time of the annealing to bedescribed later when the thickness is less than 5 nm and dead space isincreased in device design when the thickness exceeds 500 nm.Incidentally, it is also allowed to form a buffer film of polysilicon orα silicon instead of silicon nitride. Then, oxygen ions 16 are implantedinto the surface of the substrate 12 by using the mask oxide film 23 asa mask (FIG. 2( f)). The implanting condition of oxygen ions 16 is thesame as in the first embodiment. Though sputtering occurs along withoxygen ion implantation, substrate surface 12 a serving as an SOI areais covered with the buffer film 32 so that it is possible to prevent thesubstrate surface 12 a serving as an SOI area from being partiallyetched. Moreover, Though the upper margin of the mask oxide film 23 maybe deformed so as to expand above the substrate surface 12 a whenimplanting oxygen ions 16, the buffer film 32 formed on the side of themask oxide film 23 prevents the above expansion deformation so that itis possible to uniform the implantation depth of oxygen ions 16.

Then, after removing the buffer film 32 from the upside of the maskoxide film 23. More particularly the mask oxide film 23 is decreased inthickness so that the difference between thicknesses of the oxide film34 a to be formed on the substrate surface 12 a serving as an SOI areaand that of an oxide film 34 b to be newly formed on the substratesurface 12 b serving as a bulk area at the time of annealing to bedescribed later (thickness of oxide film 34 a minus thickness of oxidefilm 34 b) becomes 0.7 to 1.3 times, preferably 0.9 to 1.1 times largerthan the thickness of a buried oxide film 13 to be described later Then,as shown in FIGS. 2( g) and 2(h), this embodiment shows a case ofdecreasing the mask oxide film 23 on the surface of the substrate 12 inthickness to form a thin mask oxide film 23. This decreasing the maskoxide film 23 in thickness is performed by anisotropically etching themask oxide film 23. The anisotropic etching of the above mask oxide film23 is performed after forming a resist layer 33 on the substrate surface12 a serving as an SOI area on which the mask oxide film 23 is notformed (FIG. 2( g)). The resist layer 33 is formed in accordance withthe same procedure as in the case of the resist layer 17. Specifically,a resist layer is formed on the entire surface of the substrate 12 onwhich the buffer film 32 is formed through photolithography; the resistlayer is exposed by using a photomask and developed and rinsed; and theresist layer formed on the mask oxide film 23 is removed to leave theresist layer 33 at the side of the mask oxide film 23 and at thesubstrate surface 12 a serving as an SOI area on which the mask oxidefilm 23 is not formed.

As shown in FIG. 2( h), anisotropic etching is vertically applied to thesurface of the substrate 12 by using the resist layer 33 as a mask toremove the whole buffer film 32 from the upside of the mask oxide film23 and then the mask oxide film 23 is decreased in thickness to form amask oxide film 23 with thickness decreased as compared to the thicknessof the buried oxide film to be described later. Anisotropic etching isreactive ion etching. In this case, the thickness of the mask oxide film23 decreased in thickness is limited to the range of 0.7 to 1.3 times ofthe thickness of the buried oxide film 13 for the following reason. Whenthe thickness of the buried oxide film 13 is less than 0.7 times, thesubstrate surface 12 a serving as an SOI area on which the mask oxidefilm 23 decreased in thickness is not formed at the time of theannealing to be described later becomes higher than the substratesurface 12 b serving as a bulk area on which the mask oxide film 23decreased in thickness is formed and a state that the substrate surface12 a is present at the step or higher causes the problem of focus shiftin the photolithography step to occur. Meanwhile when the thicknessexceeds 1.3 times, the substrate surface 12 a serving as an SOI area onwhich the mask oxide film 23 decreased in thickness at the time of theannealing to be described later is not formed becomes lower than thesubstrate surface 12 b serving as a bulk area on which the mask oxidefilm 23 decreased in thickness is formed and a state that the substratesurface 12 a is present at the step or higher causes a problem of focusshift in the photolithography step to occur. After decreasing the maskoxide film 23 in thickness, the resist layer 33 is removed by a sulfuricacid/hydrogen peroxide solution or the like; the buffer film 32 isimmersed in an etching solution such as thermal phosphoric acid,hydrofluoric acid or the like, be removed, and cleaned. Incidentally, inthis embodiment, though anisotropic etching is used as means forremoving a buffer film from the upside of a mask oxide film anddecreasing the mask oxide film in thickness, it is also allowed to useisotropic etching such as wet etching as the above removing means andfilm-thickness decreasing means.

Then, the annealing where the substrate 12 is held in an oxidizingatmosphere at a temperature range between 1,300 and 1380° C. for 2 to 20hours and then slowly cooled is performed similarly to the firstembodiment (FIG. 2( j)). By this annealing, oxidation of the oxygen ionarea 20 into which oxygen ions 16 are implanted into the substrate 12 isaccelerated and the buried oxide film 13 is formed inside the substrate12. When forming the buried oxide film 13, only the substrate surface 12a serving as an SOI area is lifted due to volume expansion of the aboveoxygen ion area 20, that is, the thickness-directional expansion of theoxygen-ion area 20 and thus a step occurs between the substrate surface12 a serving as an SOI area on which the buried oxide film 13 is formedand the substrate surface 12 b serving as a bulk area on which theburied oxide film 13 is not formed as shown in FIG. 2( j). while thisstep is performed, because annealing is performed by horizontallysetting the substrate 12 in a furnace, the surface of the substrate 12is oxidized to form an oxide layer 34. Formation of the oxide layer 34is quickly progressed on the substrate surface 12 a on which the maskoxide film 23 decreased in thickness is not formed, that is, substratesurface 12 a serving as an SOI area on which the buried oxide film 13 isformed while it is slowly progressed on the substrate surface 12 b onwhich the mask oxide film 23 decreased in thickness is formed, that is,substrate surface 12 b serving as a bulk area on which the buried oxidefilm 13 is not formed. As a result, in a state that the buried oxidefilm 13 is formed, the oxidized layer 34 has the thick-layer portion 34a of the substrate surface 12 a serving as an SOI area and thethin-layer portion 34 b of the substrate surface 12 b serving as a bulkarea. Therefore, though the expansion in volume of oxygen-ion area 20due to annealing causes the substrate surface 12 a serving as an SOIarea to be lifted and thus a step is formed between the substratesurface 12 a serving as an SOI area and the substrate surface 12 bserving as a bulk area, the step is absorbed by the difference betweenthick-layer portion 34 a and thin-layer portion 34 b of the oxide layer34 formed on the substrate 12.

After forming the buried oxide film 13 by annealing, the substrate 12 isimmersed in a mixed solution (etching solution) of hydrofluoric-acidammonium aqueous solution and hydrofluoric acid to remove the mask oxidefilm 23 decreased in thickness and an oxidized layer 34 newly formed onthe surface of the substrate 12 at the time of annealing (FIG. 2( k)).Then, the step formed on the surface of the substrate 12 due to theexpansion in volume of the buried oxide film 13 almost or completelydisappears by removing the thick-layer portion 34 a and thin-layerportion 34 b of the oxidized layer 34 so that it is possible to set thestep between the substrate surface 12 a serving as an SOI area on whichthe buried oxide film 13 is formed and the substrate surface 12 bserving as a bulk area on which the buried oxide film 13 is not formedto 0 to 30 nm. By using the SOI substrate 31, it is possible to solvethe problem that a focus is shifted in the photolithograph step.

Third Embodiment

FIG. 3 shows third embodiment of the present invention. In FIG. 3, anumeral same as that in FIG. 1 denotes the same component.

This embodiment further includes a step of etching a mask oxide film 23to form a predetermined thickness and a step of forming a buffer film 42on a predetermined-width boundary area contacting with the side of themask oxide film 23 of silicon substrate surface 12 a serving as an SOIarea and on the side of the mask oxide film 23 between a step ofimplanting oxygen ions 16 and a step of performing annealing.Specifically, after implanting oxygen ions 16 (FIG. 3( e)), the maskoxide film 23 is decreased in thickness so that the difference betweenthicknesses of the oxide film 34 a formed on the substrate surface 12 aserving as an SOI area at the time of annealing to be described laterand an oxide film 34 b to be newly formed on the substrate surface 12 bserving as a bulk area (thickness of the oxide film 34 a minus thicknessof the oxide film 34 b) becomes 0.7 to 1.3 times, preferably 0.9 to 1.1times of the thickness of a buried oxide film 13 to be described laterMoreover, in this embodiment, FIGS. 3( f) and 3(g) shows a case ofdecreasing the mask oxide film 23 on the surface of a substrate 12 inthickness to form a thin mask oxide film 23. This decreasing the maskoxide film 23 in thickness is performed by anisotropically etching theabove mask oxide film 23. The anisotropic etching of the above maskoxide film 23 is performed after forming a resist layer 43 on thesubstrate surface 12 a serving as an SOI area on which the mask oxidefilm 23 is not formed (FIG. 3( f)). Formation of the resist layer 43 isperformed in accordance with the same procedure as in the case of theresist layer 17.

Specifically, the resist layer 43 is formed on the entire surface of thesubstrate 12 on which the mask oxide film 23 is partially formed throughphotolithography, the resist layer 43 is exposed by using anot-illustrated photomask and developed and rinsed to remove the resistlayer 43 formed on the mask oxide film 23 and leave the resist layer 43on the substrate surface 12 a serving as an SOI area on which the maskoxide film 23 is not formed. In this state, anisotropic etching isvertically applied to the surface of the substrate 12 by using theresist layer 43 as a mask to decrease the mask oxide film 23 inthickness. The anisotropic etching is reactive ion etching in thisembodiment. In this case, the reason why the thickness of the mask oxidefilm 23 decreased in thickness is limited to the range of 0.7 to 1.3times larger than the thickness of the buried oxide film 13 conforms tothe same reason as the case of the second embodiment. After decreasingthe mask oxide film 23 in thickness, the resist layer 23 is removed by asulfuric acid/hydrogen peroxide solution or the like (FIG. 3( g)) toclean the mask oxide film 23. Incidentally, though in this embodimentanisotropic etching is used as means for decreasing a mask oxide film inthickness, it is also allowed to use isotropic etching such as wetetching as the above film-thickness decreasing means.

Then, the buffer film 32 made of silicon nitride is formed on thesubstrate surface 12 a serving as an SOI area and the upside and theside of the mask oxide film 23. The thickness of the buffer film 32ranges between 5 and 500 nm, preferably between 20 and 200 nm. Thereason why the thickness of the buffer film 32 is limited to the rangebetween 5 and 500 nm conforms to the same reason as the case of theabove second embodiment. Incidentally, it is also allowed to form thebuffer film of not silicon nitride but polysilicon or α silicon. In thisstate, a resist layer 46 is formed. The resist layer 46 is formed wherea resist layer is formed on the entire surface of the substrate 12 onwhich the buffer film 42 is formed through photolithography, exposed byusing a photomask, developed and rinsed; a resist layer formed on themask oxide film 23 decreased in thickness and on the resist layer 43formed at the central portion of the substrate surface 12 a serving asan SOI area are removed and the resist layer 46 is left only on thepredetermined-width boundary area contacting with the side of the maskoxide film 23 on the silicon substrate surface 12 a serving as an SOIarea. The predetermined width of the above boundary area is set to 0.1to 5 μm, preferably to 0.2 to 1 μm. In this case, the width of theboundary area on which the resist layer 46 is left is limited to therange of 0.1 to 5 μm for the following reason: when the width is lessthan 0.1 μm, it is impossible to prevent oxygen from entering from theboundary area at the time of annealing and meanwhile, when the withexceeds 5 μm, dead space is increased in view of device design. As shownin FIG. 3( i), anisotropic etching is vertically applied to the surfaceof the substrate 12 by using the resist layer 46 as a mask to remove thebuffer film 42 on the upside of the mask oxide film 23 decreased inthickness and on the buffer film 42 at the central portion of thesubstrate surface 12 a serving as an SOI area. The anisotropic etchingis reactive ion etching in this embodiment. Incidentally, it is allowedto remove the buffer film on the upside of the mask oxide film decreasedin thickness and on the buffer film at the central portion of thesubstrate surface serving as an SOI area through isotropic etching suchas wet etching instead of through anisotropic etching but. As a result,the boundary area on the silicon substrate surface 12 a serving as anSOI area and the side of the mask oxide film 23 decreased in thicknessare covered with the buffer film 42 (FIG. 3( j)).

In this state, the annealing where the substrate 12 is held in anoxidizing atmosphere in a temperature range between 1,300 and 1380° C.for 2 to 20 hours and slowly cooled is performed similarly to in thefirst embodiment (FIG. 3( k)). By this annealing, oxidation of theoxygen-ion area 20 into which oxygen ions 16 in the substrate 12 areimplanted is accelerated and the buried oxide film 13 is formed insidethe substrate 12. When forming the buried oxide film 13, only thesubstrate surface 12 a serving as an SOI area corresponding to theoxygen-ion area 20 is lifted due to expansion of the above oxygen-ionarea 20, that is thickness-directional expansion in the volume and thusa step occurs between the substrate surface 12 a serving an SOI area onwhich the buried oxide film 13 is formed and the substrate surface 12 bserving as a bulk area on which the buried oxide film 13 is not formed.

While this step is performed, because annealing is performed byhorizontally setting the substrate 12 in a furnace, the surface of thesubstrate 12 is oxidized and an oxidized layer 44 is formed. Formationof the oxidized layer 44 is quickly progressed on the substrate surface12 a on which the mask oxide film 23 decreased in thickness is notformed, that is, on the substrate surface 12 a serving as an SOI area onwhich the mask oxide film 23 decreased in thickness while it is slowlyprogressed on the substrate surface 12 b serving as a bulk area on whichthe mask oxide film 23 decreased in thickness is formed, that is, on thesubstrate surface 12 b serving as a bulk area on which the buried oxidefilm 13 is not formed. As a result, the oxidized layer 44 has thethick-layer portion 44 a of the substrate surface 12 a serving as an SOIarea and the thin-layer portion 44 b of the substrate surface 12 bserving as a bulk area. Therefore, though the expansion in volume ofoxygen-ion area 20 due to annealing causes the substrate surface 12 aserving as an SOI area to be lifted and thus a step is formed betweenthe substrate surface 12 a serving as an SOI area and the substratesurface 12 b serving as a bulk area, the step is absorbed by thedifference between the thick-layer portion 44 a and the thin-layerportion 44 b of the oxidized layer 44 formed on the substrate 12.Moreover, when annealing the boundary area contacting with the maskoxide film 23 decreased in thickness of the substrate surface 12 aserving as an SOI area and the side of the mask oxide film decreased inthickness while they are exposed without being covered with the bufferfilm 42, oxygen is supplied on the peripheral portion of the buriedoxide film 13 from the upside and downside thereof as well as from theperiphery of thereof and more oxygen is supplied to the peripheralportion of the buried oxide film 13 as compared to the central portionto which oxygen is supplied from only vertical direction so thatoxidation is progressed to more extent than the central portion at thetime of annealing. As a result, the thickness of the peripheral portionof the buried oxide film 13 is increased after annealing and there is afear that the circumferential portion may be exposed on the substratesurface 12 a. In this embodiment, however, because the boundary areacontacting with the side of the mask oxide film 23 decreased inthickness of the substrate surface 12 a serving as an SOI area and theside of the mask oxide film 23 decreased in thickness are covered withthe buffer film 42, it is possible to prevent oxide from entering fromthe peripheral portion. This result in that it is possible to securelyprevent the periphery of the buried oxide film 13 from being exposed onthe surface of the substrate 12.

After forming the buried oxide film 13 through annealing, the abovesubstrate 12 is immersed in an etching solution of thermal phosphoricacid, hydrofluoric acid or the like to remove the buffer film 42 andfurther immersed in a mixed solution (etching solution) ofhydrofluoric-acid ammonium aqueous solution and hydrofluoric acid toremove the mask oxide film 23 decreased in thickness and the oxidizedlayer 44 newly formed on the surface of the substrate 12 at the time ofannealing (FIG. 3( m)). Then, the buried oxide film 13 is expanded andthe step formed on the surface of the substrate 12 is removed togetherwith the thick-layer portion 44 a of the oxidized layer 44 and when thethin-layer portion 44 b of the oxidized layer 44 is removed, it ispossible to set the step between the substrate surface 12 a serving asan SOI area on which the buried oxide film 13 is formed and thesubstrate surface 12 b serving as a bulk area on which the buried oxidefilm 13 is not formed to 0 to 30 nm. By using the SOI substrate 41, itis possible to solve a trouble that a focus is shifted in thephotolithography step.

Fourth Embodiment

FIG. 4 shows fourth embodiment of the present invention. In FIG. 4, anumeral same as that in FIG. 1 denotes the same component.

This embodiment further includes a step of forming a buffer film 52 on apredetermined-width boundary area contacting with the side of a maskoxide film 23 of silicon substrate surface 12 a serving as an SOI areaand the side of the mask oxide film 23 and a step of etching the maskoxide film 23 to a predetermined thickness between a step of implantingoxygen ions 16 and a step of performing annealing. Specifically, afterimplanting oxygen ions 16 (FIG. 4 e), the buffer film 52 made of siliconnitride is formed on substrate surface 12 a serving as an SOI area andthe upside and on the side of the mask oxide film 23. The thickness of abuffer film 52 ranges between 5 and 500 nm, preferably between 20 and200 nm. This limitation of the thickness of the buffer film 52 isrestricted in the range between 5 and 500 nm conforms to the reason sameas in the above second embodiment 2. Incidentally, it is also allowed toform a buffer film of polysilicon or α silicon instead of siliconnitride. In this state, the resist layer 53 is formed. A resist layer 53is formed where a resist layer is formed on the entire surface of thesubstrate 12 on which the buffer film 52 is formed throughphotolithography, exposed by using a photomask, developed and rinsed andthe resist layer formed on the mask oxide film 23 and the resist layer43 formed at the central portion of the substrate surface 12 a servingas an SOI area are removed, and the resist layer 53 is left only on apredetermined-width boundary area contacting with the side of the maskoxide film 23 on the silicon substrate surface 12 a serving as an SOIarea. A predetermined width of a boundary area is set to the rangebetween 0.1 to 5 μm, preferably between 0.2 and 1 μm. This limitation ofthe width of the boundary area for leaving the resist layer 53 in therange of 0.1 to 5 μm conforms to the reason that the width is the sameas in the width of a boundary area for leaving the resist layer of thethird embodiment. As shown in FIG. 4( g), anisotropic etching isvertically applied to the surface of the substrate 12 by using theresist layer 53 as a mask to remove the buffer film 52 on the upside ofthe mask oxide film 23 and the buffer film 52 at the central portion ofthe substrate surface 12 a serving as an SOI area. The anisotropicetching is reactive ion etching in this embodiment. Incidentally, it isalso allowed to remove a buffer film on the upside of the mask oxidefilm and a buffer film at the central portion of the substrate surfaceserving as an SOI area by isotropic etching such as wet etching, notanisotropic etching. As a result, the boundary area of silicon substratesurface 12 a serving as an SOI area and the side of the mask oxide film23 are covered with the buffer film 52 (FIG. 4( h)).

In this state, the mask oxide film 23 is decreased in thickness to forma thickness 0.7 to 1.3 times, preferably 0.9 to 1.1 times of thethickness of the buried oxide film 13. Then, this embodiment shows acase of decreasing the thickness of the mask oxide film 23 on thesurface of the substrate 12 to form a thin mask oxide film 23 as shownin FIG. 4 where the mask oxide film 23 is decreased in thickness bywet-etching (isotropic-etching). Specifically, the substrate 12 isimmersed in a hydrofluoric-acid aqueous solution to decrease the maskoxide film 23 in thickness. In this case, because the side of the maskoxide film 23 is covered with the buffer film 52, it is possible tolimit the etching of the side of the mask oxide film 23. The mask oxidefilm 23 is decreased in thickness and then cleaned. Incidentally, thoughthis embodiment uses wet etching (isotropic etching) as means fordecreasing a mask oxide film in thickness, it is also allowed to useanisotropic etching such as reactive etching as the above film-thicknessdecreasing means.

In this state, annealing where the substrate 12 is held in an oxidizingatmosphere in a temperature range of 1,300 to 1380° C. for 2 to 20 hoursand then slowly cooled is performed similarly to the case of the firstembodiment (FIG. 4( j)). By this annealing, oxidation of the oxygen ionarea 20 into which oxygen ions 16 are implanted in the substrate 12 isaccelerated and the buried oxide film 13 is formed inside the substrate12. When forming the buried oxide film 13, only the substrate surface 12a serving as an SOI area is lifted due to volume expansion of the aboveoxygen ion area 20, that is the thickness-directional expansion of theoxygen ion area 20 as shown in FIG. 4( j), a step is produced betweenthe substrate surface 12 a serving as an SOI area on which the buriedoxide film 13 is formed and the substrate surface 12 b serving as a bulkarea on which the buried oxide film 13 is not formed.

While the above step is performed, because annealing is performed byhorizontally setting the substrate 12 in a furnace, the surface of thesubstrate 12 is oxidized and the oxidized layer 54 is formed. Formationof the oxidized layer 54 is quickly progressed on the substrate surface12 a on which the mask oxide film 23 is not formed, that is, thesubstrate surface 12 a serving an SOI area on which the buried oxidefilm 13 is formed while it is slowly progressed on the substrate surface12 b on which the oxide film 23 decreased in thickness is formed, thatis, the substrate surface 12 b serving as a bulk area on which theburied oxide film 13 is not formed. As a result, the oxidized layer 54in a state where the buried oxide film 13 is formed has a thick-layerportion 54 a of the substrate surface 12 a serving as an SOI area and athin-layer portion 54 b of the substrate surface 12 b serving as a bulkarea. Therefore, though the expansion in volume of the oxygen ion area20 due to annealing causes the substrate surface 12 a serving as an SOIarea to be lifted and a step is produced between the substrate surface12 a serving as an SOI area and the substrate surface 12 b serving as abulk area, the step is absorbed by the difference between thethick-layer portion 54 a and the thin-layer portion 54 b of the oxidizedlayer 54 formed on the substrate 12. Moreover, when annealing theboundary area contacting with the side of the mask oxide film 23decreased in thickness on the substrate surface 12 a serving as an SOIarea and the side of the mask oxide film 23 decreased in thickness whilethe boundary area and the side are exposed without being covered by thebuffer film 52, oxygen is supplied to the peripheral portion of theburied oxide film 13 from the upside and downside of the peripheralportion as well as from oxygen is also supplied from the periphery.Therefore, more oxygen is supplied to the peripheral portion of theburied oxide film 13 as compared to the case of the central portion towhich oxygen is supplied only from vertical direction, oxidation isprogressed to more extent than the central portion, the peripheralportion of the buried oxide film 13 after annealed is further increasedin thickness as compared to the central portion, and there is a fearthat the peripheral portion may be exposed on the substrate surface 12a. In this embodiment, however, the boundary area contacting with theside of the mask oxide film 23 decreased in thickness on the substratesurface 12 a serving as an SOI area and on the side of the mask oxidefilm 23 decreased in thickness are covered with the buffer film 52 sothat the side of the mask oxide film 23 is not etched even if isotropicetching easier than anisotropic etching for decrease of the mask oxidefilm 23 in thickness without using a resist layer is used. Therefore, itis possible to prevent oxygen from entering from the boundary area. As aresult, it is possible to securely prevent the periphery of the buriedoxide film 13 from being exposed on the surface of the substrate 12.

After forming the buried oxide film 13 through annealing, the abovesubstrate 12 is immersed in an etching solution of thermal phosphoricacid, hydrofluoric acid or the like to remove the buffer film 52, andfurther immersed in a mixed solution (etching solution) of hydrofluoricacid to remove the mask oxide film 23 decreased in thickness and theoxidized layer 54 newly formed on the surface of the substrate 12 at thetime of annealing (FIG. 4( k)). Thereby the step formed on the surfaceof the substrate 12 when the buried oxide film 13 is expanded is removedtogether with the thick-layer portion 54 a of the oxidized layer 54 andwhen the thin-layer portion 54 b of the oxidized layer 54 is removed, itis possible to set the step between the substrate surface 12 a servingas an SOI area on which the buried oxide film 13 is formed and thesubstrate surface 12 b serving as a bulk area on which the buried oxidefilm 13 is not formed to 0 to 30 nm. By using an SOI substrate 51, it ispossible to solve the problem that a focus is shifted in thephotolithography step.

Fifth Embodiment

FIG. 5 show fifth embodiment of the present invention. As shown in FIG.5, the SOI substrate 61 has a silicon substrate 12 and a buried oxidefilm 13 formed inside the substrate 12. The substrate 12 is cut in asliced form along the plane perpendicular to the axis of asingle-crystal silicon rod grown through the Czochralski (CZ) method“plane (100) of crystal structure of single crystal silicon”. A buriedoxide film 13 is formed as described below. Incidentally, it is allowedto cut a substrate from a single-crystal silicon rod or single-crystalsilicon plate grown in accordance with the floating zone (FZ) methodinstead of the CZ method.

Firstly, a surface oxide film 14 is formed on the surface of thesubstrate 12 (FIG. 5( a)). The surface oxide film 14 is a silicon oxidefilm (SiO₂ film) which is formed by thermally oxidizing the substrate 12or with the CVD method (chemical vapor deposition method). The thicknessof the surface oxide film 14 is formed in the range between 200 and1,000 nm, preferably between 500 and 800 nm. this limitation ofthickness of the surface oxide film 14 in the range between 200 and1,000 nm is for the following reason: there is a fear that oxygen ions16 to be described later pass through the surface oxide film 14 and maybe implanted into the substrate 12 when the thickness is less than 200nm meanwhile it is possible to sufficiently cut off oxygen ions 16 whenthe thickness is 1,000 nm or less. Then, a resist layer 17 having apredetermined pattern is formed on the surface of the surface oxide film14 through photolithography (FIGS. 5( b) and 5(c)). The resist layer 17is exposed by using a photomask 18 (FIG. 5( b)) and developed andrinsed, and a predetermined pattern is formed on the resist layer 17(FIG. 5( c)). In the resist layer 17, a portion corresponding to theperiphery of a buried oxide film 13 to be obtained is exposed andthereby a portion corresponding to the periphery of the buried oxidefilm 13 to be obtained is removed.

Anisotropic etching is applied to the surface oxide film 14 verticallyto the surface of the substrate 12 by using the above resist layer 17 asa mask (FIGS. 5( d) and 5(e)). The anisotropic etching is reactive ionetching in this embodiment. In the reactive ion etching, though notillustrated, by mounting a substrate on the lower electrode of twoopposite electrodes set in a reactive chamber, applying a high-frequencyvoltage to these electrodes to induce plasma, a radical ion specieshaving reactivity higher than such etching gas as CF₄ or SF₆ is formed,the radical ions of several tens to several hundreds of keV enter thesubstrate 12 by the self-bias potential difference produced betweenplasma and substrate 12, and etching of the surface oxide film 14 isprogressed by the both effects of sputtering action and chemicalreaction due to the radical ions. Therefore, the inner margin of thesurface oxide film 14 becomes a vertical etching shape free fromundercut. Incidentally, as the anisotropic etching, it is allowed to useECR plasma etching. After the etching is completed, the resist layer 17is removed by a sulfuric acid/hydrogen peroxide solution or the like topartially form a contour oxide film 19 constituted of the surface oxidefilm 14 left on the surface of the substrate without being etched andhaving a thickness of 200 to 1,000 nm on the surface of the substrate 12(FIG. 5( f)) and to clean the oxide film 19. In the resist layer 17,because the portion corresponding to the periphery of the buried oxidefilm 13 to be obtained is removed through exposure, only the portion insurface oxide film 14 corresponding to the periphery of the buried oxidefilm 13 to be obtained is etched and thus the portion of the contouroxide film 19 left on the surface of the substrate, corresponding to theperiphery of the buried oxide film 13 to be obtained is opened.

Then, silicon ions 62 are implanted into the surface of the substrate 12by using the contour oxide film 19 as a mask (FIG. 5( g)). In this case,for implanting conditions of silicon ions 62 implantation quantityranges between 1×10¹⁵/cm² to 1×10¹⁸/cm², preferably 1×10¹⁶/cm² to1×10¹⁷/cm² and implantation energy ranges between 40 and 240 keV,preferably between 100 and 240 keV.

Then, the resist layer 22 is formed on the surface of a portion of thecontour oxide film 19 not corresponding to the buried oxide film 13 tobe obtained through photolithography (FIGS. 5( h) and 5(i)).Specifically, the resist layer 22 formed on the whole contour oxide film19 is exposed by using a not-illustrated photomask; a portioncorresponding to the buried oxide film 13 to be obtained is exposed andto be removed; and the resist layer 22 is left on the surface of aportion not corresponding to the buried oxide film 13 to be obtained.Then, the contour oxide film 19 is etched by using the resist layer 22as a mask (FIGS. 5( i) and 5(j)). Thereafter, the resist layer 22 isremoved by a sulfuric acid/hydrogen peroxide solution or the like (FIGS.5( j) and 5)(k) to partially form the mask oxide film 73 constituted ofthe contour oxide film 19 left on the substrate surface without beingetched on the surface of the substrate 12 (FIG. 5( k)). In this case,because the resist layer 22 is formed on the surface of a portion notcorresponding to the buried oxide film 13 to be obtained, a portion ofthe mask oxide film 73 corresponding to the buried oxide film 13 to beobtained is opened. Moreover, because detailed means for formation ofthe resist layer 22 and for etching of the contour oxide film 19 is thesame as in the above-described formation of the resist layer 17 andetching of the surface oxide film 14, repetitive description is omitted.

Then, oxygen ions 16 are implanted into the surface of the substrate 12by using the mask oxide film 73 as a mask (FIG. 5( l)). In this case,for implanting conditions of oxygen ions 16, implantation quantityranges between 1×10¹⁷/cm² and 2×10¹⁸/cm², preferably between 2×10¹⁷/cm²and 5×10¹⁷/cm² and implantation energy ranges between 20 and 200 keV,preferably between 60 and 180 keV. After implanting oxygen ions 16, themask oxide film 73 is removed from the surface of the substrate 12through wet etching to perform the annealing where the substrate 12 isin a oxidizing atmosphere in a temperature in the range between 1,300and 1,380° C. for 2 to 20 hours and slowly cooled (FIG. 5( m)). Anoxidizing atmosphere contains a mixed gas atmosphere of inert gas andoxygen and is exemplified by a mixed-gas atmosphere of argon and oxygenor mixed gas atmosphere of nitrogen and oxygen. The oxidizing atmospherein this case contains 100 vol % of oxygen. Preferable content of oxygenranges between 0.5 and 90 vol % and more preferable content rangesbetween 40 and 70 vol %. When the oxygen content is less than 0.5%,oxidation of the surface of the substrate 12 cannot be expected at thetime of annealing to be described later.

Oxidation of a portion to which the oxygen ions 16 are implanted of thesubstrate 12 is accelerated by the above annealing and the buried oxidefilm 13 is formed inside the substrate 12. At the same time, an annealedoxidized layer 26 is formed on the surface of the substrate 12. When,after forming the buried oxide film 13 by the above annealing, the abovesubstrate 12 is immersed in a mixed solution (etching solution) ofhydrofluoric-acid ammonium aqueous solution and hydrofluoric acid toremove the oxidized layer 26 (FIG. 5( n)), the SOI substrate 61 in whichthe buried oxide film 13 is formed inside the substrate 12 is obtained.

In this method for manufacturing an SOI substrate, since silicon ions 62are implanted into the surface of the silicon substrate 12 correspondingto the periphery of the buried oxide film 13 to be obtained beforeimplanting oxygen ions, the silicon ion inhibits oxidation at theperiphery of the buried oxide film 13 at the time of subsequentannealing so that it is possible to avoid the state that the thicknessis further increased as compared to that of other portion. As a result,it is possible to prevent the edge area of the buried oxide film 13 frombeing exposed on the surface of the substrate 13 due to the expansion involume of the periphery of the buried oxide film 13.

Sixth Embodiment

FIG. 6 shows sixth embodiment of the present invention. In FIG. 6, anumeral same as that in FIG. 5 denotes the same component and thusrepetitive description is omitted.

As shown in FIG. 6, the method of this embodiment in characterized infurther including a step of forming a concave groove 72 on the surfaceof a silicon substrate 12 corresponding to the periphery of the buriedoxide film 13 to be obtained before forming a mask oxide film 73 orbetween a step of forming the mask oxide film 73 and a step ofimplanting oxygen ions 16. More particularly, a silicon substrate isprepared similarly to the case of the previous embodiment to form asurface oxide film 14 on the surface of the substrate 12 (FIG. 6( a)).Then, a resist layer 17 having a predetermined pattern is formed on thesurface of a surface oxide film 14 through photolithography (FIGS. 6( b)and 6(c)) to anisotropically etching the surface oxide film 14vertically to the surface of the substrate 12 by using a resist layer 17as a mask (FIGS. 6( d) and 6(e)). After the etching is completed, theresist layer 17 is removed by a sulfuric acid/hydrogen peroxide solutionor the like and a contour oxide film 19 constituted of the surface oxidefilm 14 left on the substrate surface without being etched is partiallyformed on the surface of the substrate 12 (FIG. 6( f)) and then cleaned.In this case, the contour oxide film 19 is a film in which a portioncorresponding to the periphery of a buried oxide film 13 to be obtainedis opened.

Then, the surface of the substrate 12 on which the contour oxide film 19is not formed is lowered by a predetermined value to form a concavegroove 72 through Si etching. Though the Si etching for forming theconcave groove 72 is exemplified by a dry etching method such as areactive ion etching method or chemical dry etching method or wetetching method can be used, the dry etching method is preferable whichcan accurately form the concave groove 72 on the surface of thesubstrate 12 serving as an SOI area. It is preferable that the depth ofthe concave groove 72 is formed into a predetermined thickness in therange between 50 and 500 nm and it is more preferable that the depthranges between 100 and 300 nm. This limitation of the depth of theconcave groove 72 in the range between 50 and 500 nm is because an areaserving as a dead space on the design of a device is increased when thedepth of the concave groove 72 is less than 50 nm and because theperiphery of the buried oxide film 13 is not formed at a position farfrom the surface of the substrate 12 when the depth of the concavegroove 72 exceeds 500 nm.

Thereafter, a resist layer 22 is formed on the surface of a portion notcorresponding to the buried oxide film 13 to be obtained of a contouroxide film 19 through photolithography (FIGS. 6( h) and 6(i)) to etchthe contour oxide film 19 by using the resist layer 22 as a mask (FIGS.6( i) and 6(j)). Thereafter, the resist layer 22 is removed by asulfuric acid/hydrogen peroxide solution or the like (FIGS. 6( j) and6(k)) to partially form a mask oxide film 73 constituted of the contouroxide film 19 left on the substrate surface without being etched on thesurface of the substrate 12 (FIG. 6( k)). Then, by using the mask oxidefilm 73 as a mask, oxygen ions 16 are implanted into the surface of thesubstrate 12 (FIG. 6( l)). After implanting oxygen ions 16, the maskoxide film 73 is removed from the surface of the substrate 12 throughwet etching to perform the annealing where the substrate 12 is held inan oxidizing atmosphere in a temperature range between 1,300 and 1,380°C. for 2 to 20 hours and then slowly cooled (FIG. 6( m)). Oxidation ofthe portion to which oxygen ions 16 of the substrate 12 are implanted isaccelerated by the annealing and a buried oxide film 13 is formed insidethe substrate 12. At the same time, an oxidized layer 26 is formed bythe annealing on the surface of the substrate 12. After forming a buriedoxide film 13 through the above annealing, the substrate 12 is immersedin a mixed solution (etching solution) of hydrofluoric-acid ammoniumaqueous solution and hydrofluoric acid to remove an oxidized layer 26(FIG. 6( n)). Thereby, an SOI substrate 71 on which a buried oxide film13 is formed inside the substrate 12 is obtained.

In the method of the sixth embodiment, a concave groove 72 is formed onthe surface of the silicon substrate 12 corresponding to the peripheryof the buried oxide film 13 to be obtained before implanting oxygen ions16. Therefore, the periphery of the buried oxide film 13 obtainedthrough implantation of oxygen ions 16 is kept at a distance the surfaceof the substrate 12 along the concave groove 72 (FIG. 6( l)). Therefore,even if the thickness at the periphery of the buried oxide film isfurther increased than other portion at the time of subsequent annealing(FIG. 6( n)), the edge area at the periphery of the buried oxide film 13does not reach nor is exposed on the surface of the substrate 12.Therefore it is possible to effectively prevent the edge area from beingexposed on the surface of the substrate 12.

Seventh Embodiment

FIG. 7 shows seventh embodiment of the present invention. In FIG. 7, anumeral same as that of the above embodiments denotes the same componentand repetitive description is omitted.

As shown in FIG. 7, in the method of this embodiment, it ischaracterized in that implantation of oxygen ions 16 is performed aplurality of times separately and further includes an etching step ofetching the margin of the mask oxide film 73 between precedentoxygen-ion implanting step and subsequent oxygen-ion implanting step.

That is, a silicon substrate is prepared similarly to the case of theabove embodiments to form a surface oxide film 14 on the surface of thesubstrate 12 (FIG. 7( a)). Then, a resist layer 17 having apredetermined pattern is formed on the surface of the surface oxide film14 through photolithography (FIGS. 7( b) and 7(c)). Anisotropic etchingis applied to the surface oxide film 14 vertically to the surface of thesubstrate 12 by using the resist layer 17 as a mask (FIGS. 7( d) and7(e)). After the etching is completed, the resist layer 17 is removed bya sulfuric acid/hydrogen peroxide solution or the like to partially forma mask oxide film 73 constituted of the surface oxide film 14 left onthe substrate surface without being etched on the surface of thesubstrate 12 (FIG. 7( f)). Then, oxygen ions 16 are implanted to thesurface of the substrate 12 by using the mask oxide film 73 as a mask.This Implantation of the oxygen ions 16 is performed a plurality oftimes separately.

That is, when the final implantation quantity of oxygen ions 16 rangesbetween 1×10¹⁷/cm² and 2×10¹⁸/cm², implantation of oxygen ions at2.5×10¹⁶/cm² to 5×10¹⁷/cm² is performed two to four times. FIG. 7 showsthat implantation is performed twice (FIGS. 7( g) and 7(i)). Then, theimplantation energy at that time ranges between 20 and 200 keV,preferably ranges between 60 and 180 keV. Then, an etching step (Fig.(FIG. 7( h)) of etching the margin of the mask oxide film 73 is includedbetween the precedent oxygen-ion implanting step (FIG. 7( g)) and thesubsequent oxygen-ion implanting step (FIG. 7( i)). Though the etchingof the margin is exemplified by dry etching method such as the reactiveetching method or chemical dry etching or by wet etching method, the dryetching for accurately etching the margin of the mask oxide film 73 ispreferable. It is preferable that the margin t of the mask oxide film 73pared by the etching ranges between 10 and 500 nm, preferably between 50and 200 nm. When the margin t of the mask oxide film 73 is less than 10nm, a trouble occurs that the periphery of the buried oxide film 13 doesnot become thin. Meanwhile, when the margin t of the mask oxide film 73to be pared exceeds 500 nm, the shielding effect of oxygen ions iseliminated from the mask oxide film 73 and there is a disadvantage thatoxygen ions are implanted to the substrate 12 covered with the maskoxide film 73.

After implantation of oxygen ions 16 is performed before and after theetching step (FIG. 7(h)) a plurality of times separately (FIGS. 7( g)and 7(i)), the mask oxide film 73 is removed from the surface of thesubstrate 12 through wet etching to perform the annealing where thesubstrate 12 is held in an oxidizing atmosphere in the temperature rangebetween 1,300 and 1380° C. for 2 to 20 hours and then slowly cooled(FIG. 7( j)). Oxidation of a portion into which oxygen ions areimplanted of the substrate 12 by the annealing is accelerated and theburied oxide film 13 is formed inside the substrate 12. At the sametime, the oxidized layer 26 by annealing is formed on the surface of thesubstrate 12. After forming the buried oxide film 13 through the aboveannealing, the substrate 12 is immersed in a mixed solution (etchingsolution) of hydrofluoric-acid ammonium aqueous solution andhydrofluoric acid to remove the oxidized layer 26 (FIG. 7( k)). As aresult, an SOI substrate 81 on which the buried oxide film 13 is formedinside the substrate 12 is obtained.

In the method of the seventh embodiment, the margin of the mask oxidefilm 73 is decreased by etching the margin while oxygen ions 16 areimplanted a plurality of times. Therefore, the periphery correspondingto the decreased mask oxide film 73 of the buried oxide film 13 to beobtained through a plurality of times of implantation of oxygen ions 16becomes thin as compared to other portion. Therefore, even if thethickness at the periphery of the buried oxide film 13 is furtherincreased as compared to other portion at the time of subsequentannealing, the thickness becomes uniform with that of other portionbecause the thickness is thinner before annealing, the edge area at theperiphery of the buried oxide film 13 does not reach the surface of thesubstrate 12, and thus it is possible to prevent the edge area of theburied oxide film 13 from being exposed on the surface of the substrate12.

Eighth Embodiment

FIG. 8 shows eighth embodiment of the present invention. In FIG. 8, anumeral same as that in the above embodiments denotes the same componentand thus repetitive description is omitted.

As shown in FIG. 8, in the method of this embodiment, it ischaracterized that implantation of oxygen ions 16 is performed aplurality of times separately and a margin expanding step of expandingthe margin of the mask oxide film 73 is further included between theprecedent oxygen-ion implanting step and the subsequent oxygen-ionimplanting step.

More particularly, firstly, a silicon substrate is prepared similarly tothe precedent embodiments to form a surface oxide film 14 on the surfaceof the substrate 12 (FIG. 8( a)). Secondly, a resist layer 17 having apredetermined-pattern is formed on the surface of the surface oxide film14 through photolithography (FIGS. 8( b) and 8(c)) to apply anisotropicetching to the surface oxide film 14 vertically to the surface of thesubstrate 12 by using the resist layer 17 as a mask (FIGS. 8( d) and8(e)). After the etching is completed, the resist layer 17 is removed bya sulfuric acid/hydrogen peroxide solution or the like to partially forma mask oxide film 73 constituted of the surface oxide film 14 left onthe substrate surface without being etched on the surface of thesubstrate 12 (FIG. 8( f)). Thirdly, oxygen ions 16 are implanted intothe surface of the substrate 12 by using the mask oxide film 73 as amask. This implantation of oxygen ions 16 is performed a plurality oftimes separately (FIGS. 8( g) and 8(i)). Because implantation of oxygenions 16 performed a plurality of times separately is the same as in theabove-described third embodiment, repetitive description is omitted.

Then, a margin expanding step (FIG. 8( h)) of expanding the margin ofthe mask oxide film 73 is set between the precedent oxygen-ionimplanting step (FIG. 8( g)) and the subsequent oxygen-ion implantingstep (FIG. 8( i)). Expansion of the margin of the mask oxide film 73 isperformed by patterning a CVD-SiO₂ film. It is preferable that themargin d of the mask oxide film 73 is expanded in the range between 10and 500 nm by the margin expanding step and it is more preferable thatthe margin d is expanded in the range between 50 and 200 nm. Whenexpansion of the margin d is less than 10 nm, a trouble occurs that theperiphery of the buried oxide film 13 does not become thin and whenexpansion of the margin exceeds 500 nm, an area serving as a dead spaceis increased in device design.

After oxygen ions 16 are implanted separately a plurality of times(FIGS. 8( g) and 8(i)) before and after the margin expanding step (FIG.8( h)), the mask oxide film 73 is removed from the surface of thesubstrate 12 through wet etching and then, the annealing is performedwhere the substrate 12 is held in an oxidizing atmosphere in atemperature range between 1,300 and 1,380° C. for 2 to 20 hours andslowing cooled (FIG. 8( j)). Oxidation of a portion to in which oxygenions 16 are implanted of the substrate 12 is accelerated by theannealing and the buried oxide film 13 is formed inside the substrate12. At the same time, the oxidized layer 26 by annealing is formed onthe surface of the substrate 12. After forming the buried oxide film 13through the above annealing, the substrate 12 is immersed in a mixedsolution (etching solution) of hydrofluoric-acid ammonium aqueoussolution and hydrofluoric acid to remove the oxidized layer 26 (FIG. 8(k)). As a result, an SOI substrate 91 in which the buried oxide film 13is formed inside the substrate 12 is obtained.

In the method of the eighth embodiment while oxygen ions 16 areimplanted separately a plurality of times, the mask oxide film 73 isexpanded by applying overlay to the periphery of the film 73. Therefore,a portion corresponding to the expanded periphery of the buried oxidefilm 13 obtained through implantation of oxygen ions 16 a plurality oftimes becomes thinner than other portion of the buried oxide film 13.Therefore, even if the thickness at the periphery of the buried oxidefilm 13 is further increased than other portion at the time ofsubsequent annealing, the thickness becomes uniform with the thicknessof other portion because the above thickness is thinner beforeannealing, the edge area at the periphery of the buried oxide film 13does not reach the surface of the substrate 12, and thus it is possibleto effectively prevent the edge area of the buried oxide film 13 frombeing exposed on the surface of the substrate 12.

Ninth Embodiment

FIG. 9 shows ninth embodiment of the present invention. In FIG. 9, anumeral same as that of the above embodiments denotes the same componentand thus repetitive description is omitted.

As shown in FIG. 9, the method of this example is characterized afeature that a recess portion 23 a (FIG. 9( h)) is formed on the uppercorner of a mask oxide film 73. Specifically, a step of forming the maskoxide film 73 in this embodiment includes a step of forming a surfaceoxide film 14 on the surface of a substrate 12 (FIG. 9( a)) and a stepof forming a resist layer 17 having a predetermined pattern on thesurface of the surface oxide film 14 (FIGS. 9( b) and 9(c)). Becausethese steps are the same as the above-described third and fourthembodiments, repetitive description is omitted.

Moreover, the surface oxide film 14 is etched to obtain a mask oxidefilm 73 by using the resist layer 17 as a mask. A feature lies in thefact that the etching is performed at two stages. That is, a step offorming the mask oxide film 73 includes a step of forming the resistlayer 17 followed by performing isotropic etching to decrease thethickness of a surface oxide film 14 not masked by the resist layer 17(FIGS. 9( d) and 9(e)) and a step of vertically anisotropically etchingthe surface of the substrate 12 by using the resist layer 17 as a maskfollowed by removing the surface oxide film 14 decreased in thickness(FIGS. 9( f) and 9(g)). Anisotropic etching is performed equally in alldirections and thereby, the surface oxide film 14 is etched over thedownside of the resist layer 17. The isotropic etching includes HF wetetching and chemical dry etching. Meanwhile, anisotropic etching etchesthe surface oxide film 14 vertically to the surface of the substrate 12.Therefore, the surface oxide film 14 at a portion not covered with theresist layer 17 is etched. Thus, by removing the resist layer 17 andleaving the partially-left surface oxide film 14 on the surface of thesubstrate 12 as the mask oxide film 73, a recess portion 23 a is formedat the upper corner of the obtained mask oxide film 73 (FIG. 9( h)).

Thereafter, oxygen ions 16 are implanted into the surface of thesubstrate 12 by using the mask oxide film 73 as a mask (FIG. 9( i)).After implanting oxygen ions 16, the mask oxide film 73 is removed fromthe surface of the substrate 12 through wet etching to perform theannealing where the substrate 12 is held in an oxidizing atmosphere inthe temperature range between 1,300 and 1,380° C. for 2 to 20 hours andthen slowly cooled the substrate 12 (FIG. 9( i)). Oxidation of a portioninto which oxygen ions 16 are implanted of the substrate 12 isaccelerated by the annealing and the buried oxide film 13 is formedinside the substrate 12. At the same time, the oxidized layer 26 byannealing is formed on the surface of the substrate 12. By forming theburied oxide film 13 through the above annealing and then immersing theabove substrate 12 in a mixed solution (etching solution) ofhydrofluoric-acid ammonium aqueous solution and hydrofluoric acid toremove the oxidized layer 26 (FIG. 9( k)), an SOI substrate 101 in whichthe buried oxide film 13 is formed inside the substrate 12 is obtained.

In the method of the ninth embodiment, Since the recess portion 23 a isformed on the upper corner of the mask oxide, even if the upper marginof the mask oxide film 73 is deformed so as to expand when oxygen ions16 are implanted, the deformed portion does not protrude to a portionnot covered with the mask oxide film 73 nor change the implantationdepth of oxygen ions 16 to be implanted. Therefore, the depth of theburied oxide film 13 obtained through implantation of oxygen ions 16becomes uniform and it is possible to effectively prevent the edge areaof the buried oxide film from being exposed on the surface of thesubstrate 12.

1. A method for manufacturing an SOI substrate comprising a step ofpartially forming a mask oxide film (23) on the surface of a siliconsubstrate (12), a step of implanting oxide ions (16) into the surface ofthe substrate (12) through the mask oxide film (23), and a step ofannealing the substrate (12) to form a buried oxide film (13) inside thesubstrate (12), further comprising between the step of implanting theoxygen ions (16) and the step of annealing the substrate (12): a step ofetching the mask oxide film (23) so as to become a predeterminedthickness; and a step of forming a buffer film (42) on apredetermined-width boundary area contacting with the side of the maskoxide film (23) on a silicon substrate surface (12 a) serving as the SOIarea and on the side of the mask oxide film (23).
 2. The methodaccording to claim 1, wherein a buffer film (32, 42 or 52) is formed byany one of silicon nitride, polysilicon, and a silicon.